JP2002361928A - Imaging apparatus and imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus and an imaging method in which halftone density is settled regardless of the writing timing of a scanning line even when a multibeam is employed. SOLUTION: A decision is made whether a part being exposed simultaneously by adjacent laser beams is present or not (refer to S41) and if it is present, a decision is made whether a part adjacent to that exposing part and is not exposed by a laser beam is present or not (S43). If it is present, a microdot is exposed additionally at the part not exposed by a laser beam (S44).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using a multi-beam, and more particularly to stabilizing the density of the image forming apparatus.

[0002]

2. Description of the Related Art In a conventional image forming apparatus using an electrophotographic method in which a visible image is formed by charging, exposing, and developing, an electrostatic latent image is first charged on an electrophotographic photosensitive member as an image carrier and then charged. As a means for forming a mask, a method of performing exposure using a semiconductor laser has been widely put to practical use. The means for forming this electrostatic latent image uses a laser chip composed of a laser diode and a photodiode sensor. The output signal from the photodiode sensor is fed back to the bias power supply of the laser diode, and the amount of bias current is reduced. By performing automatic control, laser light is stabilized.

In recent years, along with high-speed printing of an image forming apparatus, means for forming an electrostatic latent image using a multi-laser for simultaneously emitting a plurality of lasers in one main scan has been put to practical use. For example, a multi-laser system using two lasers is based on the above-described configuration, and is configured by a pair of laser light emitting diodes and a photodiode sensor to stabilize laser light.

On the other hand, various image signal processing techniques for improving image quality are used. As one of them, when a digital image signal is binarized to form an image, the digital image signal is temporarily converted to an analog signal, compared with a periodic pattern signal such as a triangular wave, and subjected to pulse width modulation. A method for generating a binarized signal (PWM: PulseWi)
dth Modulation) has also been proposed. P
An invention in which the WM is applied to a multi-beam printer type laser printer is disclosed in Japanese Patent Application Laid-Open No. 8-317157. The invention of this publication is to correct pattern signals of individual lasers by pulse width modulation in order to eliminate density unevenness due to individual differences of individual lasers in a multi-beam. In other words, individual pulse width modulation may be performed on individual beams according to the characteristics of the individual beams, thereby making the light amount variations of the individual lasers uniform and equalizing the bright portion potential by the individual laser scanning. Density unevenness.

[0005]

However, there is a problem that the halftone density differs in the multi-beam even if there is no individual difference of the laser. This is a new problem that, even with the same image pattern, if the writing position is shifted by one in the sub-scanning direction, the halftone image density is different. This phenomenon is considered to be caused by the non-linearity of the curve (EV curve) between the light amount E and the potential V of the photoconductor. For example, the light intensity E at the light intensity I and the exposure time t is E = I × t. The above-mentioned density difference means that even if the same light amount E is applied to the photoconductor, if the light intensity I changes or the exposure time t changes, the sensitivity changes and the potential changes. This is called reciprocity failure. In connection with this reciprocity failure, an example in which the sensitivity is improved by repeatedly irradiating the photosensitive member with a weak light intensity several times is disclosed in JP-A-4-51.
No. 043.

The following is an example of a halftone density difference caused by reciprocity failure in a multi-beam. FIG.
Is 2dot 2sp irradiated with beams A and B simultaneously
FIG. 4 is a schematic diagram illustrating halftone of ace. A pair of lasers is defined as beam A and beam B. The beam A corresponds to the first line at the head of the writing position of the paper, and the beam B corresponds to the second line.
Corresponds to the line. Thereafter, since the beams A and B alternately correspond to each other, the beam A corresponds to an odd-numbered row, and the beam B corresponds to an even-numbered row. In the first polygon scan, beams A and B are turned on at the same time, and after scanning a 2-dot horizontal line, they are simultaneously turned off in the next polygon scan, and 2 spaces.
Can be opened. Beams A and B sequentially repeat simultaneous ON and simultaneous OFF, resulting in a two-dot two-space halftone. In FIG. 15, laser pairs for scanning a polygon are distinguished by being surrounded by a broken line at the left end.

FIG. 16 is a schematic diagram showing a two-dot two-space halftone in which beams B and A are alternately (sequentially) irradiated. In the first scan of the polygon, the beam A is turned off and the beam B is turned on, and one space is opened to scan a one-dot horizontal line. In the next polygon scan, beam A is turned on, beam B is turned off, and 1
The horizontal line of dot and 1 space are opened. This 1sp
When ace 1dot and 1dot 1space are sequentially repeated, a halftone of 2dot 2space shifted by one line is obtained.

[0008] 2 dot 2 space of FIG. 15 and FIG.
Were compared. The 2dot 2space in FIG. 15 where two lasers were simultaneously irradiated in the main scanning line direction had a density of 1.15. On the other hand, FIG.
The 2 dot 2 space of No. 6 had a concentration of 1.21. Therefore, the concentration was lower in the case of simultaneous irradiation than in the case of alternate irradiation.

In order to investigate the cause, first, it was examined whether there was a difference in light amount. Lasers interact with each other due to thermal and electrical crosstalk between the lasers,
Since it is conceivable that the light quantity may decrease during simultaneous irradiation, the laser light quantity was measured for one-pair simultaneous irradiation and for single irradiation, and a comparison was made.

FIG. 17 is a view showing a measured light amount by a pin photo when the beam A is scanned one shot. FIG.
FIG. 6 is a diagram showing a measured light amount value by a pin photo when the beam B is scanned by a single shot. FIG. 19 is a diagram showing a light intensity measurement value by a pin photo when the beam A and the beam B are simultaneously emitted and scanned. In this light quantity measurement,
The sum of the light amount of the beam A in FIG. 17 and the light amount of the beam B in FIG. 18 coincided with the light amount emitted at the same time in FIG.
From this result, it was found that the light amount of the multi-beam was stable even when it was turned on at the same time, and the light amount did not decrease.

Next, it was examined whether or not the potential of the photosensitive member was different. Because the spot diameter used this time is not small enough,
It is expected that a pair of laser spots will overlap and that the potential will be different at the overlap. The condition is that the spot diameters of the beam A and the beam B are equal, 70 μm in the main scanning direction,
It is 70 μm in the sub-scanning direction. In the body of 1200 dpi, the size of one pixel is 21 μm.

FIG. 20 is a conceptual diagram in which a light amount distribution upon simultaneous exposure is converted into a potential distribution via an EV curve.
The beams A and B are superposed on each other to generate a multi-beam light quantity, which is irradiated on the photoconductor and converted into an electric potential by an EV curve. The spot to be noted here is an overlapping portion of the spots. After the light amounts are synthesized, the light is irradiated on the photoconductor, and at the same time, a hole is generated and the potential distribution is determined. Even when the beam A and the beam B are slightly displaced in the scanning direction, it is considered that the beams are simultaneously irradiated on the photosensitive member because the time is very short, about 1 μsec.

FIG. 21 is a conceptual diagram in which a light quantity distribution upon individual exposure is converted into a potential distribution via an EV curve.
The arrow indicates a path in which a hole is generated after the first light amount of the beam A irradiates the photosensitive member, and the first potential distribution is determined.
The arrow indicates a path in which a hole is generated after the next beam B irradiates the photoconductor, and the second potential distribution is determined.

When FIG. 20 and FIG. 21 are compared, the total amount of light is the same under overlapping spots, but when irradiated simultaneously, the photoreceptor is exposed to strong light only once, and the potential is determined only once. On the other hand, even if weak light is individually irradiated, E-
Since the V-curve is non-linear with a convex downward, the potential can be sufficiently lowered, and the two potential distributions are superposed. However, the EV curve that changes the light amount here is that at the time of solid exposure, and it is not strict to apply it to a halftone of 2 dots and 2 spaces. So actually 2
The EV curve obtained by dot 2 space was measured, and it was examined whether there was a difference between simultaneous exposure and individual exposure on a non-linear photosensitive member having a downward convex shape.

FIG. 22 shows a beam A in a multi-beam.
2spa of simultaneous irradiation of laser beam and beam B and individual irradiation
FIG. 8 is a diagram showing the result of measuring the surface potential of the photoconductor by changing the amount of light at ce. In particular, Table 1 summarizes the values of 2dot 2space.

[0016]

[Table 1]

According to the graph of FIG.
The curve of the potential with respect to the light amount of the dot 2 space is always higher than the curve of the potential of the individually irradiated 2 dot 2 space, indicating that the sensitivity is inferior. In particular,
The light amount set value of the image forming apparatus is 3.0 (mJ / m ＾ 2). In the case of FIG. 15 in which a pair of beams are simultaneously irradiated, a pair of beams are individually irradiated with respect to the potential of -265 V In the case of FIG. 16, the potential was -250V. Since the density is the reversal development, the potential of -265 V is equal to the potential of -250 V
The concentration becomes lower. As described above, this difference in potential is a difference in density between 1.15 and 1.21. Therefore, in order to match the density of the individual irradiation, it is necessary to increase the light amount of the simultaneous irradiation to about 9/8 to make it equivalent to 3.4 (mJ / m ＾ 2).

As described above, in the case of a multi-beam, 1
Due to reciprocity failure of the photoreceptor, the potential was higher and the sensitivity was lower than when individually exposed due to reciprocity failure of the photoreceptor, even when the amount of light was the same when the paired beams were exposed simultaneously and individually. . In other words, when the multi-beams are emitted simultaneously in the overlapping portion of the multi-beams, the light amounts of the respective beams are superimposed and then irradiated onto the photoconductor at once. On the other hand, when the multi-beams individually emit light in the overlapping portion of the multi-beams, the light amounts of the respective beams are individually applied to the photoconductor. At this time, the writing of the halftone image is shifted by one line, but there is a problem that the former is lower in sensitivity than the latter and has a difference in density.

The present invention has been made under such circumstances, and even when a multi-beam is used, the halftone density is stable irrespective of the writing start timing of the scanning line. It is an object to provide a forming method.

[0020]

In order to achieve the above object, according to the present invention, an image forming apparatus is configured as in the following (1) to (5), and an image forming method is performed in the following (6), (5). Configure as described in 7).

(1) In an image forming apparatus for simultaneously exposing a photosensitive member to form an image by simultaneously scanning a plurality of laser beams, it is determined whether or not there is a portion where adjacent laser beams are simultaneously exposed so as to partially overlap. 1 determining means;
When the first discriminating means determines that there is a portion to be exposed so as to partially overlap at the same time, a second discriminating device for discriminating whether there is a portion adjacent to the exposed portion and not exposed by the laser beam. And control means for controlling, when the second discriminating means determines that there is a portion not exposed by the laser beam, to add a fine dot exposure to the portion not exposed by the laser beam. Forming equipment.

(2) In an image forming apparatus in which a plurality of laser beams are simultaneously scanned to expose and expose a photosensitive member to form an image, adjacent laser beams are simultaneously exposed so as to partially overlap each other, and successive laser beams are sequentially used. To determine whether there is a part to be exposed so as to partially overlap
And the second discriminating means for judging whether there is a portion which is adjacent to the portion to be exposed so as to partially overlap at the same time and which is not exposed by the laser beam when the first discriminating portion judges that there is the portion. Means, and control means for controlling, when the second discriminating means determines that there is a portion not exposed by the laser beam, to add exposure of minute dots to the portion not exposed by the laser beam. An image forming apparatus comprising:

(3) In an image forming apparatus provided with a latent image forming means for simultaneously exposing a photosensitive member to form a latent image by simultaneously scanning two laser beams in the main scanning direction, the latent image forming means comprises a main scanning device. When exposure is performed so that the two laser beams are partially overlapped on the photoreceptor in one scan in the main scanning direction, fine dots may be formed before the one scan in the main scanning direction or in the next scanning in the main scanning direction. An image forming apparatus that adds exposure.

(4) In an image forming apparatus provided with a latent image forming means for forming a latent image by exposing a photoreceptor by simultaneously scanning two laser beams, the latent image forming means is provided only once in the main scanning direction. In the case where the two laser beams are emitted simultaneously in the scanning, exposure of minute dots is added before one scanning in the main scanning direction or in the next scanning in the main scanning direction, and one scanning in the main scanning direction is performed. Said 2 which emits light in
An image forming apparatus that does not add fine dot exposure when one of the two laser beams is adjacent to one of the two laser beams that emit light in the next scan in the main scanning direction.

(5) In the image forming apparatus described in the above (3) or (4), the minute dots have a size of 1/4 pixel or 1/8 pixel in a space of 1 pixel. An image forming apparatus having two dots.

(6) An image forming method for an image forming apparatus in which an image is formed by exposing a photosensitive member by simultaneously scanning a plurality of laser beams, wherein there is a portion where adjacent laser beams are simultaneously exposed so as to partially overlap. When it is determined in step A that there is a portion to be exposed so as to partially overlap at the same time in step A, it is determined whether there is a portion adjacent to the exposed portion and not exposed by the laser beam. Step B and step B
And C) adding fine dot exposure to the portions not exposed by the laser beam when it is determined that there is a portion not exposed by the laser beam.

(7) An image forming method in an image forming apparatus for exposing a photosensitive member to form an image by simultaneously scanning a plurality of laser beams, wherein a portion to be exposed by adjacent laser beams so as to partially overlap at the same time; Step A for judging whether there is a portion to be exposed so as to be partially overlapped with the adjacent laser beam sequentially, and, if it is judged that there is a portion to be exposed so as to be partially overlapped at the same time, Step B for determining whether there is a portion not exposed by the laser beam, and, if it is determined in Step B that there is a portion not exposed by the laser beam, exposure of a minute dot is added to the portion not exposed by the laser beam. Step C to control
An image forming method comprising:

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to examples of a laser printer. Note that the present invention is not limited to the form of the apparatus, but can be implemented in the form of a method, supported by the description of the embodiment.

[0029]

Embodiment 1 FIG. 4 is a sectional view showing a schematic configuration of a "laser printer" according to Embodiment 1. FIG. In the figure, M
Denotes a printer body. The photosensitive drum 1 is a cylindrical electrophotographic photosensitive member, and is driven by an arrow R by driving means (not shown).
It is driven to rotate in one direction. On the surface of the photosensitive drum 1, a charging member 2 which is disposed in contact therewith and rotates in the direction of arrow R2.
After the charging, the latent image forming unit 3 forms an electrostatic latent image. The developing unit 4 includes a hopper as a toner storage device for storing and storing the toner T, and a developing sleeve 4 a as a toner carrier, and develops an electrostatic latent image formed on the photosensitive drum 1. . In the vicinity of the developing sleeve 4a rotating in the direction of arrow R4, a developing blade 4b as a toner regulating member is provided. Then, an AC bias is superimposed on a DC bias between the photosensitive drum 1 and the developing sleeve 4a by an engine control unit 8 having a power supply for driving the laser printer and a high-voltage circuit for supplying a bias for forming an image. Is applied. Thereby, the electrostatic latent image on the photosensitive drum 1 is
The toner is applied and developed as a toner image. The toner image on the photosensitive drum 1 is transferred onto a transfer material P such as paper by the transfer unit 5 rotating in the direction of arrow R5. The transfer material P is stored in a paper feed cassette 9, fed by a feed roller (not shown), and sent to a transfer nip between the photosensitive drum 1 and the transfer unit 5. The toner image transferred to the transfer material P is conveyed to the fixing unit 7 together with the transfer material P, where it is heated and pressed to be fixed on the transfer material P to form a recorded image.
On the other hand, after the transfer of the toner image, the toner remaining on the photosensitive drum 1 without being transferred to the transfer material P (hereinafter referred to as transfer residual toner) is transferred to the cleaning blade 6 in the cleaning unit 6.
a. The photosensitive drum 1 from which the transfer residual toner on the surface has been removed is used for the next image formation starting from the charging of the charging member 2, and the above-described series of image forming processes is repeated.

FIG. 3 is a diagram showing details of the latent image forming section in the laser printer of the present embodiment. In FIG. 3, a laser beam emitted from a semiconductor laser 21 is made substantially collimated by a collimator lens 22 and a diaphragm 23,
The light enters the rotary polygon mirror 24 with a predetermined beam diameter. The rotary polygon mirror 24 rotates at a constant angular velocity in the direction of the arrow, and the laser light incident with this rotation is reflected as a deflection beam whose angle continuously changes. The deflected laser light is condensed by the f-θ lens 25. The f-θ lens 25 simultaneously corrects distortion so as to guarantee the temporal linearity of scanning, and scans the photoconductor 1 at a constant speed in the direction of the arrow (this scanning with laser light is called main scanning). ), An electrostatic latent image is formed by turning on / off the laser beam.

The semiconductor laser 21 used here is a chip provided with two laser diodes, and can form two rows of latent images in one main scan. This pair of lasers,
Beam A and beam B are defined. The condition is that the beam A and the beam B have the same spot diameter, 70 μm in the main scanning direction and 70 μm in the sub-scanning direction. In the 1200 dpi body,
The size of one pixel is 21 μm. The beam A corresponds to the first line at the beginning of the paper writing position, and the beam B corresponds to the second line. Note that the head of the writing position is the head of the image printable area, and the head line does not change even if it is blank. Thereafter, since A and B alternately correspond to each other, beam A
Corresponds to odd rows, and beam B corresponds to even rows.

Next, a method and a circuit for adding a minute pixel of the semiconductor laser 21 to form a halftone line having a uniform density will be described.

FIG. 1 is a block diagram showing an example of a pulse width control circuit of a semiconductor laser. The pulse width control circuit is
Image classification circuit 3 for classifying odd and even rows of image data
0, a memory 31 for storing a light emitting position in the main scanning direction,
A dot position control circuit 32 for generating a minute dot signal;
PWM for generating a triangular wave based on a minute dot position signal
The circuit 33 includes a beam A lighting circuit 34 and a beam B lighting circuit 35 for controlling the beam lighting of the semiconductor laser.

In the pulse width control circuit configured as described above, the image classification circuit 30 determines image data in which adjacent main scanning data exists and a pair of beams emit light simultaneously. First, the image classification circuit 30 classifies a pair of main scan data into odd rows of the beam A and even rows of the beam B. In the same scan, when there is data in the beam A and the beam B, the light emission position in the main scanning direction is stored in the memory 31.
To memorize. If there is no data of the beam A in the next scan, the beam A does not emit light conventionally. However, in this embodiment, the dot position control circuit 32 sends a signal to the beam A lighting circuit 34 at this stored position (the position where simultaneous light emission was performed in the previous scan) even if there is no data, and a minute dot is emitted. Let it. That is, the operation timing of the dot position control circuit 32 is during the next scanning beam A corresponding to the position where the emission of the beam A and the emission of the beam B coincide with each other, and there is no data in this beam A. At this time, a dot position signal is sent to the PWM circuit 33. The PWM circuit 33 is
A triangular wave synchronized with the reference clock is generated based on the dot position signal and sent to the beam A lighting circuit. By the modulation with the triangular wave, the beam A lighting circuit 34 emits a minute dot of 画素 of one pixel of the beam A, and can increase the halftone density.

FIG. 2 is a flowchart showing a process of classifying image data by the image classification circuit. The pair of main scanning data is classified into odd rows of the beam A and even rows of the beam B. Adjacent 2n-1 and 2n rows (n = 1,2,2
3) are the same scans, and it is determined whether or not the positions in the scanning direction of the data of the 2n-1th row and the 2nth row match (S4).
1, the same applies hereinafter). If Yes, the light emission position in the main scanning direction is stored in the memory 31 (S42). If No, the process returns to the start and waits for the next image data. Next, 2n + 1
It is determined that the data of the beam A in the row is not at the light emitting position of the memory 31 (S43). In the case of Yes, the image classification circuit 30 activates the dot position control circuit 32, and the beam A
To emit 1/4 minute dots of one pixel (S4
4). If No, the process returns to the start and waits for the next image data. This is because, because the beam A emits light according to the data, it is not necessary to emit fine dots. Finally,
The same process is repeated as long as the simultaneous light emission position continues (S4
5).

FIG. 5 is a diagram showing an example of a time chart when there is a dot position signal (ON) and when there is no dot position signal (OFF).

On the left side of the time chart, the beam A lighting circuit 34 modulates the beam A according to the dot position signal ON, and adds a minute dot of 1/4 pixel to increase the halftone density. The memory 31 in the image classification circuit 30 stores a simultaneous light emission data position of the beams A and B along the main scanning immediately before the beam A that emits a minute dot. Based on the position data, a pulse generation position signal is sent from the image classification circuit 30 to the PWM circuit 33 via the dot position control circuit 32. PWM circuit 3
3 generates a triangular wave synchronized with the reference clock based on the pulse generation position signal. If the beam A has an image signal in advance, the image signal is given priority over the minute dots, and the pulse generation position signal is not transmitted.

In the beam A lighting circuit 34, the reference pixel clock, the triangular wave from the PWM circuit 33, and the lighting signal of the beam A lighting circuit 34 modulated on the basis of the triangular wave are synchronized with each other, and are adjusted to a minute dot width. A modulated lighting signal is obtained. Further, when the time of the image clock cycle corresponding to one pixel is divided into eight, the lighting signals are lit at the fourth and fifth positions from the top as shown in FIG. The eighth is thinned out.

On the other hand, the right side of FIG.
4 is a time chart when there is no dot position signal (OFF). On the right side of this time chart, the beam A is not modulated in the beam A lighting circuit 34 due to the dot position signal OFF, and no fine dots are generated.
It should be noted that this position does not correspond to the memory 31 in the image classification circuit 30, and the beam A,
B does not emit light simultaneously.

Next, a lighting pattern corresponding to the time chart of FIG. 5 will be described with reference to FIG. The left side of FIG.
After the simultaneous irradiation of the beams A and B by the dot position signal, the minute dots are additionally lit. The lighting time of the minute dot corresponds to 1/4 pixel, and is shown below and adjacent to the beam B in FIG. On the right side of FIG. 6, since the beams A and B immediately before are not turned on, the minute dots are not turned on.

FIG. 7 is a part of a schematic diagram of 2 dots 2 spaces when the beams A and B are simultaneously irradiated. The main scanning line is from left to right, and the beam A and the beam B of the previous pair are adjacent to each other in a pair of scans. The minute dots of the beam A of the next set are lit at a rate of 1/4, and when the upper and lower scans are combined, the laser emission time is increased by 1/8 to 2 dots 2 spaces. This is calculated from FIG. 22 because it is sufficient to increase the light quantity from 3.0 to 3.4 (mJ / m ＾ 2) in order to match the density of the individual irradiation. Of 9
About / 8.

FIG. 8 is a part of a schematic diagram of a 2 dot 2 space in which the beam B and the next beam A are individually irradiated. Both horizontal lines of the beam B and the beam A are fully lit at 8/8. At this time, since the dots are individually lit, the dot position control circuit 32 does not operate, and no minute dots are added.

FIG. 9 is a diagram showing the results of measuring the surface potential of the photoreceptor by changing the amount of light in 2 dots and 2 spaces of simultaneous irradiation of the beams A and B and individual irradiation. The surface potential of the photoreceptor was measured by changing the amount of light in order to confirm the effect of the simultaneously lit fine dots. Here, the density of a halftone image formed by repeating 2 dots and 2 spaces at 1200 dpi and the latent image potential on the drum were measured. This is shown in Table 2.

[0044]

[Table 2]

The conditions are the simultaneous irradiation of the beam A and the beam B and the 2 dot 2 space where the minute dots are added and the 2 dot 2 space where the individual irradiation is performed. From this graph, it can be seen that the curve of the potential with respect to the light quantity of 2dot 2space irradiated at the same time is lower than the addition of the fine dots and almost coincides with the curve of the potential of 2dot 2space of the individual irradiation. 2dot 2space obtained by adding microdots to the simultaneous irradiation has a density of 1.21, and 2dot 2space individually irradiated has a density of 1.21.
21. As described above, the density was also matched by the effect of the minute dots.

As described above, in the case of the multi-beam, when comparing a pair of beams simultaneously exposed and individually exposed, the simultaneous exposure has a higher potential than the individual exposure and the sensitivity is inferior. The potential can be made uniform by adding microdots in a set scan. By aligning the potential, the toner is developed, so that the density can be made equal.

There was no adverse effect on the image, such as edge tailing, for the addition of minute dots. This is 1200dp
One pixel of i is 21 μm, and the fine dot shown in FIG. 7 is 5 μm. 70μ spot diameter in main scanning direction
m, and 5 μm is sufficiently small, so that the image does not look like a trail when considering the overlap of the latent images. actually,
When the image was developed with a toner having a particle size of 7 μm, the image was not formed such that the horizontal line was trailing on the image.

(Embodiment 2) In this embodiment, a minute dot is added before a line when a pair of semiconductor lasers are irradiated simultaneously. This makes it possible to correct the halftone latent image potential without being disturbed even in an apparatus in which tailing due to development is likely to occur. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals.

FIG. 10 shows a 2 dot 2 spac when the beams A and B of this embodiment are simultaneously irradiated, which is modified from FIG.
It is a part of schematic diagram of e. The main scanning line is from left to right, and the beam A and the beam B are adjacent to each other in a pair of scans. The minute dots of the beam B in the front group are illuminated at a rate of 1/4, and when the upper and lower scans are combined, the laser emission time is increased by 1/8 to 2dot 2.
space. In the latent image, fine dots were added before the horizontal line, and the potential was corrected to -250V. That is,
The same potential correction effect as in Example 1 in which minute dots were added after the horizontal line was obtained.

(Embodiment 3) In this embodiment, two fine dots are further added to a line when a pair of semiconductor lasers are irradiated simultaneously.

FIG. 11 shows that there is a dot position signal (ON).
It is a figure showing an example of a time chart of a case and a case without (OFF). On the left side of the time chart, the beam A lighting circuit detects the beam A
, And two minute dots of 1/8 pixel are added to increase the halftone density. The memory 31 in the image classification circuit 30 stores a simultaneous light emission data position of the beams A and B along the main scanning immediately before the beam A that emits a minute dot. Based on the position data, a pulse generation position signal is sent from the image classification circuit 30 to the PWM circuit 33 via the dot position control circuit 32. PWM circuit 3
3 generates a triangular wave synchronized with the reference clock based on the pulse generation position signal. If the beam A has an image signal in advance, the image signal is given priority over the minute dots, and the pulse generation position signal is not transmitted.

In the beam A lighting circuit, a reference pixel clock, a triangular wave from the PWM circuit 33, and a lighting signal of the beam A circuit 34 modulated based on the triangular wave are synchronized, and are modulated to a minute dot width. A lighting signal is obtained. When the time of the image clock cycle corresponding to one pixel is divided into eight, the lighting signal is lit at the third and seventh positions from the top as shown in the figure. The remaining first to second, fourth to sixth, and eighth are thinned out.

On the other hand, the right side of FIG. 11 is a time chart when there is no dot position signal in the beam A lighting circuit (OFF). On the right side of this time chart, the beam A lighting circuit does not modulate the beam A due to the dot position signal OFF, and no fine dots are generated. In addition,
This position does not correspond to the memory 31 in the image classification circuit 30, and the beams A and B along the main scan immediately before the beam A are not simultaneously emitted.

Next, a lighting pattern corresponding to the time chart of FIG. 11 will be described with reference to FIG. On the left side of FIG. 12, fine dots are additionally lit after the simultaneous irradiation of the beams A and B by the dot position signal. The lighting time of the minute dot corresponds to two 1/8 pixels, and is shown below adjacent to the beam B in FIG. On the right side of FIG. 12, the minute dots do not light because the beams A and B immediately before do not light.

FIG. 13 is a part of a schematic diagram of 2 dots 2 spaces when the beams A and B are simultaneously irradiated. The main scan line is from left to right, and in a pair of scans,
This is when the beam A and the beam B of the previous set are adjacent to each other. The next set of two small dots of the beam A are lit at a rate of 1/8, two per pixel.
Laser emission time increased by 1/8 to 2dot 2spa
ce. This is calculated from FIG. 22 because it is sufficient to increase the light quantity from 3.0 to 3.4 (mJ / m ＾ 2) in order to match the density of the individual irradiation. About 9/8.

FIG. 14 is a part of a schematic diagram of a 2 dot 2 space in which the beam B and the next beam A are individually irradiated. Both horizontal lines of the beam B and the beam A are fully lit at 8/8. At this time, since the dots are individually lit, the dot position control circuit 32 does not operate, and no minute dots are added.

In the third embodiment, as in the first embodiment, in the case of a multi-beam, comparing the case where a pair of beams are simultaneously exposed and the case where they are individually exposed, the simultaneous exposure has a higher potential than the individual exposure. To solve the problem of poor sensitivity, the potential can be made uniform by adding minute dots in the next set of scans. By aligning the potential, the toner is developed, so that the density can be made equal.

In addition, there was no adverse effect on the image such as edge tailing for the addition of minute dots. This is 1200dp
One pixel of i is 21 μm, and the fine dot shown in FIG. 13 is 2.5 μm. Since the spot diameter in the main scanning direction is 70 μm, and 2.5 μm is sufficiently small, the image does not look like a trail when considering the overlap of the latent images. Actually, when the toner was developed with a toner having a particle diameter of 7 μm, an image in which a horizontal line was trailing on the image was not obtained.

Although the first, second, and third embodiments have been described using the example of a half-tone of 2 dots and 2 spaces, the present invention provides a method of forming a minute dot into a line by multi-laser simultaneous exposure and individual exposure. It proposes a method of adding before and after, and is not limited to 2 dots. Table 3 shows examples of other halftones.

[0060]

[Table 3]

In one pair of multi-beams, there are two combinations of dots. Example 1 is a dot in which the beam A is turned on in the first scan. Example 2 is a dot in which the beam A is turned off in the first scan. The example shows the first scan, the second scan, and the third scan. (OFF) is a minute dot only when Space is 1 Line.

In a pair of multi-beams, halftones may be shifted in the writing start position, and are divided into two types depending on the difference in the writing start position. Example 1 is a case where the image starts from the beam A, and Example 2 is a case where the image starts from the beam B while the beam A is OFF. In 1 dot, the potential is the same when only the beam A is fully lit or only the beam B is fully lit.

According to the flowchart of FIG. 2, when light is emitted simultaneously in one scan, in the first and third embodiments, the minute dot is turned on in the beam A of the next scan. On the other hand, Example 2
Then, the light is illuminated by the beam B before the simultaneous light emission.

In Table 3, the positions of the beams on the photosensitive member are indicated by arrows. ,, Indicate the order of scan scanning. For example, 2do of Example 1 or Example 3
In the case of the first example of t, the arrows are in the simultaneous row indicating the simultaneous scanning and in the separate row individually indicating the scanning of the next minute dot.
→ “Beam A minute dots” → “Beam B OFF”. In Example 1 and Example 3, 2d
Example 2 of ot explained that the potential was the same as Example 1.

Similarly, in the case of the two-dot example 1 of the second embodiment, the arrows are located on the individual lines indicating the minute dots of the individual scan and on the line of the simultaneous lighting, and are similarly changed from "beam B minute dot" to "beam B minute dot". This indicates that the arrangement is “beam A full lighting” → “beam B full lighting” → “beam A OFF”. Ona (OFF) indicates that the empty space is 1 line
Only in the case of, the dot is replaced by the minute dot.

From Table 3, 3dots are 1dot and 2d
ot is a combination of 4dots, 2dots and 2dos
It can be seen that this is a combination of t. Similarly, 4 dots or more are composed of a combination of 1 dot and 2 dots. Accordingly, no matter which halftone is used, the potential can be equalized even if the write start position is shifted by the method of the embodiment.

In each of the above embodiments, a latent image is formed on a photoreceptor by using two laser beams that scan simultaneously. However, the present invention is limited to the number of laser beams. However, the present invention can be applied to an image forming apparatus using more laser beams such as four or eight laser beams. And
If more than two laser beams are used, when there is a portion to be exposed so as to partially overlap with two or more laser beams that are scanned at the same time, exposure of minute dots is added by another laser beam that is scanned at the same time. You may.

[0068]

As described above, according to the present invention, even when a multi-beam is used, the halftone density is stabilized irrespective of the writing start timing of the scanning line.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a pulse width control circuit of a semiconductor laser according to a first embodiment.

FIG. 2 is a flowchart illustrating processing of an image classification circuit.

FIG. 3 is a diagram illustrating details of a latent image forming unit.

FIG. 4 is a sectional view showing a schematic configuration of the first embodiment.

FIG. 5 is a time chart corresponding to the presence or absence of a dot position signal.

6 is a schematic diagram of a lighting pattern corresponding to the time chart of FIG.

FIG. 7 is a schematic view showing the addition of simultaneous irradiation fine dots.

FIG. 8 is a schematic diagram of individual irradiation.

FIG. 9 is a diagram showing an effect of adding a minute dot.

FIG. 10 is a schematic diagram of addition before simultaneous irradiation of fine dots in the second embodiment.

FIG. 11 is a time chart corresponding to the presence or absence of a dot position signal in the third embodiment.

FIG. 12 is a schematic diagram of a lighting pattern corresponding to the time chart of FIG. 11;

FIG. 13 is a schematic view showing the addition of simultaneous irradiation fine dots.

FIG. 14 is a schematic diagram of individual irradiation.

FIG. 15 is a schematic diagram of simultaneous irradiation in a conventional example.

FIG. 16 is a schematic view of individual irradiation in a conventional example.

FIG. 17 is a diagram showing a light amount measurement value when the beam A is scanned by a single shot;

FIG. 18 is a diagram showing a measured light amount value when the beam B is scanned by a single shot.

FIG. 19 is a diagram showing measured light amounts when beams A and B are simultaneously scanned.

FIG. 20 is a conceptual diagram showing a potential distribution when beams A and B are simultaneously scanned.

FIG. 21 is a conceptual diagram showing a potential distribution when beams A and B are individually scanned.

FIG. 22 is a diagram showing the surface potential of the photoconductor when the simultaneous irradiation of the beams A and B and the individual irradiation are performed while changing the light amount.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photosensitive drum 21 semiconductor laser 30 image classification circuit 31 memory 32 dot position control circuit 33 PWM circuit 34 beam A lighting circuit 35 beam B lighting circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 1/113 G03G 15/04 120 5C071 / 23 103 (72) Inventor Seiichi Shinohara Shimomaruko, Ota-ku, Tokyo 3-30-2 Canon Inc. (72) Inventor Takayuki Namiki 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term (reference) 2C362 AA07 BA56 CA12 CA13 CA14 2H027 DA50 EA02 EC11 EC20 ED06 ED07 EE07 EF09 2H076 AB06 AB75 AB76 2H106 AA12 AA41 AA73 AB02 5C072 AA03 BA17 CA06 HA02 HA06 HA10 HB08 XA05 5C074 AA10 BB03 CC22 DD06 EE02 GG12 HH02 HH04

Claims (7)

[Claims] An image forming apparatus for simultaneously exposing a photosensitive member to form an image by simultaneously scanning a plurality of laser beams to determine whether there is a portion to be exposed so that adjacent laser beams partially overlap at the same time. When it is determined that there is a portion to be exposed so as to partially overlap at the same time by the first determining unit and the first determining unit, it is determined whether there is a portion adjacent to the exposed portion and not exposed by the laser beam. A second discriminating means, and a control means for controlling, when the second discriminating means determines that there is a portion not exposed by the laser beam, to add a fine dot exposure to the portion not exposed by the laser beam. An image forming apparatus comprising: 2. An image forming apparatus for exposing a photosensitive member and forming an image by simultaneously scanning a plurality of laser beams, wherein an adjacent laser beam simultaneously exposes a portion to be partially overlapped and an adjacent laser beam sequentially exposes the portion. First determining means for determining whether there is a portion to be exposed so as to partially overlap, and a laser which is adjacent to the portion to be exposed so as to partially overlap at the same time when the first determining means determines that the portion is to be exposed. A second discriminating means for discriminating whether or not there is a portion which is not exposed by a beam; An image forming apparatus, comprising: control means for controlling so as to add exposure. 3. An image forming apparatus comprising: a latent image forming means for simultaneously exposing a photosensitive member to form a latent image by simultaneously scanning two laser beams in a main scanning direction; In the case where the exposure is performed so that the two laser beams partially overlap each other on the photosensitive member in one scan, the exposure of minute dots is performed before one scan in the main scan direction or in the scan in the next main scan direction. Image forming apparatus. 4. An image forming apparatus provided with a latent image forming means for simultaneously exposing a photosensitive member to form a latent image by simultaneously scanning two laser beams, wherein the latent image forming means performs one scanning in a main scanning direction. In the case where the two laser beams simultaneously emit light, exposure of minute dots is added before one scan in the main scan direction or in the next scan in the main scan direction, and in one scan in the main scan direction, When one of the two laser beams that emit light and one of the two laser beams that emit light in the next scan in the main scanning direction are adjacent to each other, no fine dot exposure is added. An image forming apparatus comprising: 5. The image forming apparatus according to claim 3, wherein the minute dots are 1 dot of 1/4 pixel size or 2 dots of 1/8 pixel size in one pixel space. An image forming apparatus, comprising: 6. An image forming method for an image forming apparatus for exposing a photosensitive member to form an image by simultaneously scanning a plurality of laser beams, wherein there is a portion where adjacent laser beams are exposed so as to partially overlap at the same time. And determining if there is a portion to be exposed so as to partially overlap at the same time by the step A, and determining whether there is a portion adjacent to the exposed portion and not exposed by the laser beam. B, and, if it is determined in step B that there is a portion not exposed by the laser beam, a step C of adding fine dot exposure to the portion not exposed by the laser beam. Method. 7. An image forming method in an image forming apparatus for exposing a photosensitive member to form an image by simultaneously scanning a plurality of laser beams, wherein an adjacent laser beam is exposed so as to partially overlap at the same time and an adjacent portion is exposed. Step A for determining whether there is a portion to be exposed so as to partially overlap with the laser beam to be sequentially irradiated. If it is determined in step A that there is a portion to be exposed, the laser beam is adjacent to the portion to be exposed to partially overlap at the same time. A step B of determining whether there is a portion not exposed by the beam; and, if it is determined in the step B that there is a portion not exposed by the laser beam, exposure of minute dots is added to the portion not exposed by the laser beam. An image forming method, comprising: controlling step C.